Abstract
Humic substances in groundwater are expected to affect the migration behavior of radionuclides by forming soluble colloid complexes with radionuclides. In order to understand the interaction between radionuclides and humic substances, it is also important to estimate the effect of the competing cations originally present in the groundwater. In this connection, the complex formation of Fe(II) with Aldrich humic acid has been investigated by ion exchange method. To evaluate the effect of heterogeneous composition and of polyelectrolyte nature of humic acid, the complex formation of Fe(II) with homogeneous polymeric weak acid, polyacrylic acid (MW = 90000 Dalton) has been also examined to compare the effect of pH, ionic strength and metal-ion concentration on the complex formation. By defining the apparent formation constant as βα = [ML]/([M][R]), where [M] and [ML] are the concentrations of free and bound Fe2+ ion, and [R] = CRα (CR is the total concentration of proton exchange sites and α is the degree of dissociation of humic or polyacrylic acid), the values of logβα have been obtained at pcH 4.6 to 5.5 in 0.1 and 1.0 M NaCl at the concentration of Fe(II) from ～10-8 to ～10-4 M. For both humate and polyacrylate complexes, logβα increased with the degree of dissociation and decreased with ionic strength (at I = 0.1 M NaCl, from 2.26 (α = 0.32) to 2.59 (α = 0.47) for Fe(II)-polyacrylate and from 4.66 (α = 0.58) to 4.83 (α = 0.70) for Fe(II)-humate, and at I = 1.0 M NaCl, from 0.53 (α = 0.49) to 0.98 (α = 0.71) for Fe(II)-polyacrylate and from 3.31 (α = 0.59) to 3.62 (α = 0.71) for Fe(II)-humate). The dependence of logβα of humate on logα was similar to that of polyacrylate and that of humate on ionic strength was slightly less than that of polyacrylate. While the variation of Fe(II) concentration had no appreciable influence on logβα of Fe(II)-polyacrylate, logβα of Fe(II)-humate appreciably increased at lower Fe(II) concentrations due to the heterogeneous composition of the humic acid (at I = 0.1 M NaCl, α = 0.68, from 3.21 (CFe = 4×10−4 M) to 4.79 (CFe = 6×10−8 M) and at I = 1.0 M NaCl, α = 0.68, from 2.35 (CFe = 4×10−4 M) to 3.58 (CFe = 6×10−8 M)).
